1. Field of the Invention
This invention relates to cladding of elongated metal elements.
2. Description of the Prior Art
One commercially common form of elongated clad metal product comprises copper clad aluminum wire, or rod. One conventional use of such clad wire is as coaxial cable core wire wherein the high frequency signal is carried by the thin clad copper coating and low frequency power current is carried by the entire wire. Another example of such conventional clad wire usage is in conventional construction power wiring where the coating may be formed of copper to provide good connectability and where the core may be formed of aluminum to provide relatively low cost. General specifications have been promulgated by ASTM for such products having both 10% and 15% cooper-to-aluminum ratios by volume, with the aluminum comprising an EC grade aluminum.
Different methods have been conventionally employed for producing such clad wire theretofore. In such methods, strips of copper of semitubular shape are placed about an aluminum rod. The composite is then heated with the edges of the copper strips being pressure welded in a suitable roll and with the excess copper being shaved off upon completion of the pressure welding operation. The composite rod is then drawn to a final desired diameter. This method permits the temperature of the metals to be maintained below the temperature in which intermetallic compound formation normally occurs, and more specifically, with aluminum and copper as the metals of the clad wire, the temperature is maintained below approximately 500.degree. F. One advantage of this process is the ability to utilize different alloys of copper and aluminum as desired in the final product, and one disadvantage of the method is the requirement of a relatively large copper-to-aluminum ratio.
Another disadvantage is the requirement of relatively close tolerances to assure proper copper welds, and effectively prevent aluminum "showthrough" and poor surface quality.
Another process conventionally utilized in forming such clad wire utilizes a cleaned aluminum rod which is continuously delivered into a copper tube which is formed from copper strips and welded by an inert gas shielded arc. The weld is scarfed and the tube is subsequently reduced in diameter to the size of the aluminum rod in a suitable rolling die. A subsequent approximate 80% reduction is effected by drawing steps requiring the use of relatively low yield strength aluminum rod.
A further conventional process for forming clad wire utilizes coextrusion of both the core material and the sheath material. The process is effected by a hydrostatic press wherein aluminum ingot is extruded into a formed copper tube with the core and sheath portions being bonded together as they extrude through the die opening at relatively low temperatures. This process has the disadvantage of being discontinuous and relatively expensive relative to the high cost of material preparation and low material recovery.
Still another process utilized conventionally for forming clad wire comprises an electroplating process wherein copper is plated on aluminum core material in a continuous method. A problem in this method arises from the use of tin, zinc, or similar metals in the plating substrates which promotes diffusion and brittle intermetallic layers at the copper annealing temperatures, thereby preventing annealing of the plated rod. Alternatively, the core material may be plated at more near the final desired size of the clad wire. This approach has the serious disadvantage of requiring relatively expensive plating equipment presenting serious control problems. One problem attendant the use of plating processes is the forming of nodules in the plating which may create "noise" where the wire is to be used for coaxial purposes.
Still another proposed method of forming such clad wire utilizes aluminum rod and copper tubing formed into spirals which are threaded together and drawn down to the desired final size.
Illustrative of the prior art utilizing aluminum rods covered with thin sheets of copper which are welded about the rod are the U.S. Pat. Nos. 3,600,790 and 3,565,677 of Paul A. Dion et al, and 3,408,727 of Paul A. Dion.